A levitated object is interesting to observe and can be useful in various applications. Magnetic fields provide one tool which can be used for levitating magnetic objects. According to Earnshaw's theorem, it is not possible to support a magnetic object in a stable manner using only static magnetic fields. One statement of Earnshaw's theorem is that a levitated magnet cannot be held in stable equilibrium by any combination of static magnetic or gravitational forces.
Whitehead, U.S. Pat. No. 5,168,183 discloses a magnetic levitation system in which an arrangement of magnets on one side of a separation plane can support a levitated magnetic element on an opposite side of the separation plane. The magnetic arrangement provides a static magnetic field such that, for displacements of the levitated element in directions parallel to a stability plane, the potential energy of the magnetic interaction between the levitated element and the static magnetic field increases. The magnetic arrangement includes a control system which controls a variable force, which may be a variable magnetic field, to stabilize the levitated element in a direction perpendicular to the stability plane.
The inventors have determined that the magnetic fields generated by the embodiments disclosed by Whitehead tend to apply torque to levitated magnetic elements. This is because the magnetic dipole of the levitated magnetic element aligns generally with the static magnetic field while the variable magnetic field used to control the position of the levitated magnetic element in the “unstable” direction is directed generally perpendicularly to the static magnetic field at the equilibrium location of the levitated magnetic element. The rotation caused by this torque can lead to combined torsional and translational oscillation of the levitated element. In some cases this adversely affects stable feedback control and thus the stability of the levitated magnetic element.
There is a need for magnetic levitation systems of the general type disclosed by Whitehead which have desirable operating characteristics, such as increased stability of the levitated magnetic element, and/or are simple in construction. For certain applications, there is a particular need for such levitation systems which incorporate a reduced amount of magnetic material and can be made at relatively low cost.